Development and optimization of ME-TVC desalination system

doi:10.1016/j.desal.2009.06.026

Desalination

Volume 249, Issue 3, 25 December 2009, Pages 1315-1331

https://doi.org/10.1016/j.desal.2009.06.026 Get rights and content

Abstract

Significant and rapid developments have taken place recently in the multi-effect thermal vapor compression (ME-TVC) desalination system of SIDEM Company, particularly in enlarging the unit capacity. The new trend of combining ME-TVC with conventional multi-effect has allowed this unit capacity to increase from two to eight million imperial gallons per day (MIGD) in the last decade. This considerable increase in capacity, poses a real competition to the multi stage flash system (MSF) as a large-scale production plant with lower operation temperatures.

A steady state mathematical model of the ME-TVC desalination system is developed in this paper using Engineering Equations Solver (EES) to evaluate the model system performance. The model validity is examined against three commercial ME-TVC units which showed good results. The main improvements in the performance during the past ten years are also outlined and discussed. Another purpose of this paper is to determine the optimum operating and design conditions of the ME-TVC desalination system through mathematical modeling optimization. A MATLAB algorithm solution is developed and used to solve model equations, where a different number of effects were tested to maximize the gain ratio using (1) Smart Exhaustive Search Method and (2) Sequential Quadratic Programming. Results showed that the maximum gain ratio varied between 8.5 and 18.5 for 4 and 12 effects with the optimal top brine temperature ranging between 55.8 and 67.5 °C and a reasonable specific heat transfer area. The optimal ranges of compression and entrainment ratios are between 1.81 to 3.68 and 0.73 to 1.65 respectively. The optimal results of 4-effect TVC unit are also compared with three commercial 4-effect units having almost the same input, which showed that further improvement in the distillate output production, compression and entrainment ratio can be achieved by combining the ME-TVC system with conventional multi-effect unit.

Introduction

Rapid developments have occurred recently in the Multi-Effect Thermal Vapor Compression (ME-TVC) desalination system, which makes this technology competitive enough to the multi stage flash (MSF) desalination system. It becomes available in large units up to 8 MIGD; operates at low top brine temperatures around 60 °C, has high gain ratio (GR) up to 16 and lower expenditures (Table 1). The strong competition between manufacturers led to improved designs based on technical optimization along with their experiences in the previous projects [2].

The French company SIDEM commissioned several plants of Multi Effect (ME) desalination system since 1890 [3]. Lately, it developed the thermal vapor compression system in several projects around the world, particularly in the United Arab Emirates (UAE). The first two ME-TVC units were introduced in 1973 at Das Island in UAE with a 125 m³/d unit capacity; each unit consisted of two effects. The unit capacity increased to 1500 m³/d in 1979 where four units were installed in Ruwais Refinery [4]. The first ME-TVC desalination unit of 1 MIGD capacity was commissioned in the remote western areas of UAE in December 1991 at Jabal Dhana and Sila, followed by 2 units of 1 MIGD capacity at Mirfa. Each of these units had four effects with gain ratio close to 8. A boiler was used to supply the motive steam at 25 bars [5]. The next unit capacity was 2 MIGD which started up in 1995 in Sicily (Italy). It consisted of four identical units; each had 12 effects, with gain ratio of 16. The steam was supplied from two boilers at 45 bars to the plant [6]. Due to the adequate performances of the plants, more units of 1, 1.5 and 2 MIGD were ordered from SIDEM and commissioned in UAE between 1996 and 1999 [4]. The next range in size was achieved with two units, each of 3.5 MIGD in 2000 in Umm Al-Nar and 14 units of 3.77 MIGD in 2002 in Al-Taweelah A₁. Each unit had nine effects with gain ratio close to 8. The steam was extracted from a steam turbine at 2.8 bars to supply two steam ejectors in each unit [7]. The next unit that was commissioned was in Layyah with a nominal capacity of 5 MIGD using medium pressure motive steam [4]. The largest unit ever built to date for ME-TVC was 8 MIGD, which was built for a contract with Sharjah Electricity and Water Authority in 2005 [8]. Now, this technology is starting to gain more market shares in Saudi Arabia and SIDEM has been selected to build one of the largest ME-TVC desalination plants with a total capacity of 176 MIGD, (6.5 MIGD × 27 units) in Al-Jubail Industrial City [9].

Although several studies have been published concerning ME-TVC desalination system in literatures, to the best of our knowledge, optimization of ME-TVC desalination system has not been tackled through mathematical modeling, and thus a mathematical model will be developed in this paper for designing a ME-TVC unit using a simple optimization procedure. The MATLAB program will be used to determine the optimum operating and design conditions of a different number of effects to maximize the gain ratio using two methods: (1) Smart Exhaustive Search Method and (2) Sequential Quadratic Programming. The results of this optimization will be compared with some of the existing ME-TVC plants through some case studies. The main improvements in ME-TVC will also be outlined and discussed.

Section snippets

Process description

A simplified schematic diagram of a ME-TVC desalination system with n effects is presented in Fig. 1. It consists of (1) a steam jet ejector which acts as a thermal compressor, (2) horizontal falling film evaporators (effects), (3) distillate, feed, condensate and brine disposal pumps to circulate the streams, (4) an end condenser, (5) feed heaters and (6) flashing boxes. The motive steam D_s is directed at relatively high pressure P_s into the steam ejector. Part of the vapor formed in the last

Mathematical model

A mathematical model of the ME-TVC desalination system (Fig. 1) is presented in this section. The model is developed by applying mass and energy conservation laws to the evaporators, steam ejector, feed heaters and end condenser. The following assumption were used to simplify the analysis: steady state operation, negligible heat losses to the surrounding, equal temperature difference across feed heaters, salt free distillate from all effects and variations of specific heat as well as boiling

Development of ME-TVC

Several approaches have taken place towards the improvement of multi effect desalination system in the last three decades. One of main points of development is using the falling film horizontal tube evaporators in order to increase the heat transfer coefficient which reduces the required heat transfer area [16]. The other point for enhancement is keeping the top brine temperature in the range of 60 °C and this decreases the potentiality of scale formation [4]. The most recent achievement in

The optimization approach

The previous simplified mathematical model of ME-TVC desalination system in Section 3 along with the schematic diagram in Fig. 1 is used for optimization purposes in this section. The schematic diagram consists of n number of effects varying from 4 to 12. In any mathematical optimization, the objective function, design variables and constrains should be specified in order to formulate the problem properly and to select the appropriate optimization method [21]. The general statement of the

Results and discussion

The optimal computed results of the mathematical optimization problem are displayed below in Table 6, where all the feasible solutions of the Smart Exhausted Search Method are listed in Appendix IIA, and the optimal results of the SQP method are listed in Appendix IIB.

In the light of the results shown in Table 6 the following facts can be reported:

1)
The number of effects is limited in ME-TVC system by the temperature difference between the discharged steam from the ejector and the cooling

Conclusions

This paper outlines the performance developments in multi-effect thermal vapor compression systems during the last decade. A MATLAB algorithm was developed and used to solve a mathematical model optimization problem, where different numbers of effects were tested to maximize the gain ratio using: (1) Smart Exhaustive Search Method and (2) Sequential Quadratic Programming. The maximum gain ratio varied between 8.5 and 18.5 for 4 and 12 effects with an optimal top brine temperature ranging

Glossary

A: Heat transfer area, m²
A_c: Condenser Heat transfer area, m²
A_d: Specific available energy consumption, kJ/kg
A_f: Feed heater heat transfer area, m²
At_d: Specific heat transfer area, m²/kg
B: Brine flow rate, kg/s
BPE: Boiling point elevation, °C
C: Specific heat capacity of water, kJ/kg. K
CG-ST: Combined gas-steam turbine system
CR: Compression ratio
D: Distillate, kg/s
D_f: Non entrained vapor, kg/s
D_r: Entrained vapor to steam ejector, kg/s
D_s: Motive steam flow rate, kg/s
D_s/D_r: Entrainment ratio
ER: Expansion ratio
F: Feed flow rate,

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Development and optimization of ME-TVC desalination system

Abstract

Introduction

Section snippets

Process description

Mathematical model

Development of ME-TVC

The optimization approach

Results and discussion

Conclusions

Glossary

Desalination

Desalination

Desalination

UAE. Desalination

Desalination

Desalination

Desalination

Trans IChemE

Desalination

Desalination

Desalination

Desalination